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If you put in less than the minimum octane level consistently, the check engine light may go on, indicating a fault code has been thrown up....could be P1384 - I've had this happen on my 328 and diagnosed it with by OBD II reader. You will only know this (and what it means) if you have a OBD II reader that can tell you the fault code and re-set it, otherwise the light may just stay on. And it may cost you a trip to the dealer to get it checked out and turned off - and that won't be cheap. Less than minimum octane will ultimately cause engine knock but could affect performance too. Not worth it.

Always 93 but occasionally when its running super low I'll throw some VP100 or MS109 in there and drive the hell out of it for a while.

Does the ECU respond well to increased octane? I know that not all engines will advance timing and spark beyond what 93 will allow.

My Volvo R responded VERY well to increased octane levels. Guys would run Torco additive, which worked well but left a reddish film on the tailpipes...which means who knows where else it was leaving a film.

Guys running race gas used to call it "cracktane", lol. The added pull was noticeable.

Does the ECU respond well to increased octane? I know that not all engines will advance timing and spark beyond what 93 will allow.

My Volvo R responded VERY well to increased octane levels. Guys would run Torco additive, which worked well but left a reddish film on the tailpipes...which means who knows where else it was leaving a film.

Guys running race gas used to call it "cracktane", lol. The added pull was noticeable.

It does seem to enjoy the race fuel, when the local track opens up I'd like to test and see if it really does add power or if it's a placebo effect but subjectively I'd say it makes a difference.

Colorado only offers 85, 87, and 91. Ralph Schomp BMW said 87 works fine because of our elevation, but I only run 91.

Low octane works in a normally aspirated motor at higher altitudes, but not in a turbocharged motor. I'm sure that the car can and will compensate for low octane in that it will not knock, but your performance will suffer.

I have one local station with NO ETHANOL 91; if I can't there. by 2nd choice is Shell 93 Octane (has ethanol in my area). Noticed a huge difference in performance with the non-ethanol on my 2010 535I, not quite as much on on the 2011 535I.

Your manual should be specific about whether 93 is "recommended" or "required" - there's a crucial difference. For the vast majority of cars these days, onboard electronics are programmed to compensate for lower octane fuel, technically making 93 octane obsolete for all but the highest end of sports cars. Various tests by car mags and consumer organizations have confirmed that lower octane fuel has no/neglible adverse effects (performance, wear, emissions or mpg) on the vast majority of new cars.

That said, I could've sworn I felt a slight performance difference when using 89 and 91 fuel in my wife's Audi (a 3L supercharged engine with 333hp). It was ever so slight, but I could've sworn I felt a tiny difference. That combined with unfounded doubt in my mind, made me switch back to 93. I figured the marginal incremental cost annually of 89 vs 91 vs 93 didn't warrant the lower grade fuel. I only use 93 in the M5, but would be comfortable with lower octane fuel in a non-M BMW if BMW only "recommends" 93.